The invention relates to a device for regulating (controlling) the buckling of a rigid underwater pipe, particularly one for carrying fluids such as hydrocarbons, of the rigid single or double-wall (“pipe-in-pipe”) type, with or without thermal insulation.
The problem to be overcome is that of the buckling of the rigid hydrocarbon pipes, under the effect of the axial compressive loads arising from the combined effect of the temperature and pressure of the fluid flowing in the pipe, and the hydrostatic pressure.
Above a certain threshold, these loads can cause the buckling of the rigid pipe.
Two forms of buckling are generally distinguished:                upheaval buckling, in which the pipe becomes detached from the ground and buckles upwards.        lateral buckling, in which the pipe remains on the ground and buckles to the side.        
It is important to prevent damage due to the buckling of a rigid pipe which has to undergo large variations of temperature and pressure. It is also important to prevent the buckling or expansion of the pipe from causing damage to the equipment connected to its ends, such as manifolds or crossings.
There are known devices intended to stop the propagation of buckling along an underwater pipe, the said buckling being possibly initiated during the laying of the pipe because of excessive loads in the length of catenary pipe linking the seabed to the laying ship. U.S. Pat. No. 4,300,598, U.S. Pat. No. 3,860,039, GB 1564621, U.S. Pat. No. 5,458,441 and EP 0100099 describe such devices. These devices do not resolve the problem which has arisen.
One known solution consists in burying the pipe in a trench excavated in the ground. This makes lateral buckling impossible, since the pipe is blocked on both sides by the trench. Vertical buckling is still possible, but it can be prevented if the weight of the fill is sufficient. This solution is also uneconomical and is difficult to implement in deep seas (1000 m or deeper).
Another solution consists in the use of expansion elements (flexible pipe links called “jumpers”) positioned along the pipe and intended to absorb the axial loads.
GB 2287297 describes a solution consisting in the deliberate deformation of the pipe during laying in such a way as to create regular alternating small bends (in the form of waves) along the length of the pipe. The pipe thus offers less resistance to bending and to small lateral displacements, enabling the compressive loads to be released in a uniform way along the length of the pipe, and preventing buckling and concentrations of loads. This solution is also costly because of the wavelike deformation which has to be created on the laying ship with the aid of special tools.
Other solutions, intended to control (regulate) buckling to ensure that its effects are acceptable, have been the subject of conferences reported in the following publications:
OTC (Offshore Technology Conference) 2003—King Flowlines—Thermal Expansion Design and Implementation.
OMAE (Offshore Mechanics and Arctic Engineering Conference) 1989—Thermal buckling of pipelines close to restraints.
The basic idea common to these solutions is that of carefully distributing a plurality of lateral buckling areas along the pipe, so as to effectively release the axial compressive loads, while ensuring that this buckling remains acceptable. In practice, these solutions use various devices for initiating lateral buckling, permanently installed at the desired positions. Since buckling is more easily initiated in straight sections, it has been envisaged that the pipe could be deliberately bent in the areas in which the development of buckling is desired.
Another solution consists in the local reduction of the weight of the pipe, using permanently fitted buoys: the buoys can be in the form of clamps installed during laying. These buoys reduce the apparent weight of the pipe and thus the lateral friction with the ground. Additionally, the increase in diameter at the positions of the buoys creates a discontinuity of the bending inertia, which in itself is sufficient to promote buckling.
Another known solution for initiating lateral buckling by reducing the lateral friction between the pipe and the ground consists in placing between the sea bed and the pipe transverse sliding beams or cross-members on which the pipe can easily slide laterally.
These beams are generally formed from large-diameter metal tubes. The reduction in lateral friction is partly due to the detachment of the pipeline at its intersection with the beam (absence of ground contact) and partly due to the low coefficient of friction between the pipe and the beam.
These solutions intended to initiate and control lateral buckling do not satisfactorily resolve the problem which has been stated. On the one hand, the initiation of buckling is not totally reliable. On the other hand, these solutions do not make it possible to control the shape (the mode) and amplitude of the buckling and consequently the mechanical stresses in the pipe. Finally, they cannot ensure the reproducibility of the buckling, in respect of its shape and position, throughout the temperature and pressure cycles applied to the pipe.
Lastly, in spite of the large number of proposed solutions, it is found that the problem of effectively regulating the buckling of a pipe laid on a sea bed is still unresolved.
U.S. Pat. No. 3,466,881 discloses a collar limiting the horizontal or vertical displacements of a pipe within the collar. U.S. Pat. No. 3,955,599 discloses a device for imparting a localized curvature to a pipe.
Document WO 03 012327 discloses a device limiting the bending of a flexible pipe.